This image is part of a series that was created for an open textbook, Aerodynamics and Aircraft Performance. The textbook is available here.
Kindred Grey (2021). "A Balloon as a Simple Jet." CC BY 4.0. Adapted from James F. Marchman (2004). CC BY 4.0.
As the ballon's total mass changes due to the air exiting through the hole, the gas exits with a velocity cap V sub exit to the right, resulting in a net thrust force to the left. These are perpendicular to the balloon's downward weight force.